1. Field of the Invention
The present invention relates to a seal structure for a wire-cut electric discharge machine, and more particularly to a seal structure that restricts flowage to the outside of machining fluid from an opening in a side wall of a machining tank that permits the passage of a lower arm therethrough.
2. Description of the Related Art
As is well known, in a wire-cut electric discharge machine, machining is carried out by supporting a wire electrode between upper and lower arms and causing an electric discharge between a workpiece and the wire electrode, in a state in which the workpiece is immersed in machining fluid. The machining fluid is contained in a machining tank, and an opening that permits the passage of the lower arm is provided in a side surface of the machining tank. In order to prevent the loss of machining fluid to the outside through this opening, a seal structure that restricts the flowage of machining fluid through the opening to the outside is provided around the opening.
FIG. 1 is a diagram illustrating the typical basic construction of this seal structure, with a periphery of the opening for the lower arm shown in a sectional view. Also depicted in FIG. 1 is a coordinate system for describing directions of movement. The +Z direction of this coordinate system corresponds to the vertical direction, and the XY plane corresponds to a horizontal plane perpendicular to the pull of gravity. It should be noted that descriptions of directions in terms of the aforementioned coordinate system is also used for convenience with respect to other drawings as well.
In FIG. 1, reference numeral 1 indicates a machining tank, in a side part of which is provided an opening 2 for the passage of a lower arm 10 therethrough as indicated by the dotted line, with a small amount of room to spare. This small amount of play permits simultaneous movement of the lower arm 10 in the ±X direction and the ±Y direction when moving a workpiece relative to the wire electrode (neither of which is shown) in the ±X direction and the ±Y direction. If some sort of seal structure is not provided at this opening 2, the machining fluid inside the machining tank (consisting mainly of water or oil and the like) will drain to the outside.
Accordingly, as shown in FIG. 1, a seal structure is provided that uses a seal 3 (first seal means) and a seal plate 4 (second seal means). The seal 3 (first seal means) is provided on the periphery of the opening 2 and the seal plate 4 (second seal means) is pressed against the seal 3 so as to cover the opening 2. In addition, a hole 41 that allows the lower arm 10 to pass through is formed in substantially the center of the seal plate 4 (second seal means). It should be noted that, although not directly related to the present invention, a well-known seal unit (for example, a bellow-shaped seal) 42 is provided also at the hole 41, permitting the lower arm 10 to pass in and out while doing its best to prevent leakage of machining fluid from a gap between the outer periphery of the lower arm 10 and the inner wall of the hole 41.
Thus, as described above, the seal plate 4 permits unrivalled movement of the lower arm 10 in the ±Y direction. However, the seal plate 4 cannot but hamper movement of the lower arm 10 in the ±X direction. Consequently, the seal plate 4 is pressed against the seal 3 using a suitable affixing means (for example, something that uses a spring member) while supporting the plate 4 in that affixed state in such a way as to enable the seal plate 4 to slide in the ±X direction. In addition, the length of the seal plate 4 in the ±X direction is designed to be sufficiently longer than the length of the opening (slot) 2 in the ±X direction so as to keep both edges of the seal plate 4 in the ±X direction from separating from the seal 3 while the seal plate 4 slides in the ±X direction.
The problem with this sort of seal structure is leakage of the machining fluid from between the seal plate 4 and the seal 3. The amount of leakage can be reduced if the force of attachment of the affixing means 5 is very great, thus pressing the seal plate 4 hard against the seal 3. However, doing so increases the frictional force acting between the seal plate 4 and the seal 3 and prevents the seal plate 4 from sliding smoothly.
The conventional solution to this problem is to form a so-called labyrinth seal structure where the seal 3 contacts the seal plate 4 so as to reduce the amount of leakage of machining fluid.
FIG. 2 is a plan view of the basic form of the seal 3 employed conventionally, as seen from the ±Y direction in FIG. 1. As shown in FIG. 2, an unevenly shaped part composed of two ridges and one groove is provided on the side where the seal 3 contacts the seal plate 4 (see FIG. 1) so as to form a labyrinth structure for suppressing machining fluid leakage. The entire seal 3 is shaped like a frame along the periphery of the opening 2 (see FIG. 1), with reference numerals 31a-31d indicating top, bottom, left and right grooves, respectively, as seen from the +Y direction. As for the two ridges, an inner ridge is indicated by reference numeral 32a and an outer ridge is indicated by reference numeral 32b. 
Although it is true that using a seal 3 of such a construction can reduce the amount of leakage of the machining fluid to some extent, as a practical matter, it is virtually impossible to eliminate completely the leakage of machining fluid using such a seal structure. Consequently, a certain amount of flowage of machining fluid to the outside of the machining tank is considered inevitable, and therefore efforts to alleviate this problem are currently limited to providing means for recovering the machining fluid that thus drains away.
Here, with respect to where the leakage of machining fluid occurs, in general, it may be thought that the greatest leakage would likely occur at the bottom (the −Z side), where the pressure head is greatest. In reality, however, experience shows that, as shown by the numerous arrows 6 shown in FIG. 2, leaks can occur on all sides. Particularly when machining fluid leaks from the top and the sides, it sometimes develops into a leak in which the machining fluid creeps along the seal plate 4 (see FIG. 1) to the outside of the machine itself. If such a machining fluid leakage outside the machine occurs, it can degrade the surrounding environment and require that the machine be shut down.
Such a problem is difficult to solve using the unevenly shaped labyrinth seal structure like that disclosed for example in JP 3026120B for the seal. In addition, although a conventional technique of providing a wiper-like member on the sides of the seal unit to prevent leakage to the outside the machine in order to prevent machining fluid leakage from the top and the sides of the seal and along the seal plate 4 from leaking outside of the machine, is well known, such an arrangement has the disadvantage that the structure of the seal unit becomes complicated, inviting an increase in cost.
Thus, as described above, with existing seal structures the leakage from the seal unit is not consistent or cannot be identified, and therefore a complicated or large-scale collection structure that must be able to cope with leaks from all locations on the seal unit is required to collect the machining fluid that leaks from the machining tank.